His main challenger -- former military ruler Muhammadu Buhari -- was a contestant in the past elections in 2003 and 2007. Buhari is the candidate for the Congress for Progressive Change.

Other candidates include Nuhu Ribadu and current Kano state Gov. Ibrahim Shekarau.

A CNN iReporter in Lagos, who gave her name as Jan Young, said she expected the race to be tight.

"If the incumbent president wins, it won't be a landslide victory but a fair split between the ruling and opposition parties who campaigned for our votes. I also expect our nation would demand accountability from whoever wins at the end of the day," she wrote.

The winner will face tough challenges in the former British colony that has been mostly under military rule in its 50 years of independence.

For starters, to avoid a runoff, the winner must get at least a quarter of the vote in two-thirds of the 36 states and the capital.

The ruling party has a tradition of alternating power between the mostly Muslim north and the Christian and animist south after two terms in office.

The rhythm was broken when Jonathan, a southerner, succeeded Yar'Adua during his second term in office. Some ruling party leaders have demanded a candidate from the north.

Saturday's voting was largely peaceful, except for a bomb that exploded in northeastern Nigeria early in the day. The blast was in Maiduguri city in Borno state, according to a spokesman for the National Emergency Management Agency.

Gunfire also rang out in that city, the spokesman said, but there were no immediate reports of casualties.

The relative calm stood in contrast to the violence that marred the country's parliamentary elections last week. During that vote, separate bomb blasts ripped through a polling station and a collation center in northeastern Nigeria.

Human Rights Watch has estimated that at least 85 people have been killed in political violence so far.

A new election chief promised free and fair elections, but the electoral commission was forced to put off elections earlier this year by a week after logistical problems, including party logos missing from ballot papers, were reported nationwide.

It was a major setback reminiscent of the nation's 2007 elections, which the European Union described as filled with rampant vote rigging, violence, theft of ballot boxes and intimidation.

Nigeria, Africa's most populous country and its largest oil producer, is a major supplier of crude oil to the United States, and hosts many Western oil companies and workers.

Nigerians voted April 9 for 360 House of Representatives seats and 109 Senate seats. After the presidential election on Saturday, a gubernatorial vote will be held on April 26.

Nigerians are working to harness the power of the internet this year to demand accountability and encourage citizens to get involved in the voting process.

The west African nation has 43 million internet users -- the largest in Africa -- and they are increasingly using Twitter, Facebook and blogs to report cases of voting problems during the election season.